THE PLACENTA 125 a special sucking mouth. This sucking mouth is an extra-uterine production, and is of course an adaptation to the particular needs of the young, just as are other larval organs, such as the chin- suckers of the tadpole, or the regular ciliated bands of the larvae of various marine invertebrate organisms. There are a number of other features which distinguish the Marsupials from other mammals. The cloaca of the Marsupials is somewhat reduced, but is still recognisable. Its margins in Tarsipes are even raised into a wall, which projects from the body. The tooth series of the Marsupials was once held to consist of one dentition only, with the exception of the last premolar, which has a forerunner. The interpretation of the teeth of Marsupials are various. Perhaps most authorities regard the teeth as being of the milk dentition, with the exception of course of the single tooth that has an obvious forerunner. But there are some who hold that the teeth are of the permanent dentition. In any case it is proved that a set of rudimentary teeth are developed before those which persist. Those who believe in the persisting milk dentition describe these as prelacteal. Another matter of im- portance about the teeth of this order of mammals is that their numbers are sometimes in excess of the typical Eutherian 44. This, however, holds good of the Polyprotodonts only, It was for a long time held that the Marsupials differed from all other mammals in having no allantoic placenta. But quite recently this supposed difference has been proved to be not universal by the discovery in jPerameles of a true allantoie placenta. The Marsupials have been sometimes called the JDi- delphia. This is on account of the fact that the uterus and the vagina are double- Very frequently the two uteri fuse above, and from the point of junction an unpaired descending passage is formed (see Fig. 48 on p. 74). A character of the brain of Marsupials has been the subject of some controversy. Sir Richard Owen stated many years ago that they were to be distinguished from the higher mammals by the absence of the corpus callosum. Later still it was urged that a true corpus callosum, though a small one, was present; while, finally, Professor Symington1 seems to have shown that 1 **Th.e Cerebral Commissures in tlie Marsmpialla and Monotretnata," Anat. Phys. xxvii. 1893, p. 69.